TITLE: GCN CIRCULAR NUMBER: 22822 SUBJECT: Konus-Wind observation of GRB 180618A (a short GRB with extended emission) DATE: 18/06/22 10:32:51 GMT FROM: Dmitry Svinkin at Ioffe Institute D. Svinkin, S. Golenetskii, R. Aptekar, D. Frederiks, M. Ulanov, A. Tsvetkova, A. Lysenko, A. Kozlova, and T. Cline on behalf of the Konus-Wind team, report: The bright, short-duration, hard GRB 180618A (Swift-BAT detection: LaPorte et al., GCN Circ 22790; Fermi-GBM detection: Hamburg et al., GCN Circ 22794) triggered Konus-Wind at T0=2591.201 s UT (00:43:11.201). The burst light curve shows a bright multi-peaked initial pulse with a total duration of ~0.250 s, followed by an extended emission seen up to ~T0+10 s. The emission of the initial pulse is seen up to ~10 MeV. The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB180618_T02591/ As observed by Konus-Wind, the burst had a fluence of 6.24(-1.13,+1.20)x10^-6 erg/cm2, and a 16-ms peak flux, measured from T0-0.010 s, of 5.60(-1.83,+1.88)x10^-5 erg/cm2/s (both in the 20 keV - 10 MeV energy range). The time-averaged spectrum of the initial pulse (measured from T0 to T0+0.192 s) is best fit in the 20 keV - 10 MeV range by a power law with exponential cutoff model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with alpha = -0.36(-0.24,+0.30) and Ep = 2461(-565,+788) keV (chi2 = 47/38 dof). Fitting by a GRB (Band) model yields the same alpha and Ep, and an upper limit on the high energy photon index: beta < -2.4 (chi2 = 47/37 dof). The time-averaged spectrum of the extended emission (measured from T0+0.256 to T0+8.448 s) is best fit in the 20 keV - 2 MeV range by the CPL model with alpha = -0.68(-0.79,+1.59), Ep = 170(-66,+203) keV (chi2 = 102/98 dof). Among ~300 bursts from the Konus catalog of short GRBs (Svinkin et al., 2016, ApJS 224, 10), GRB 180618A is within top 1% in the terms of 16-ms peak energy flux and within top 15% in the terms of the total energy fluence, both in the 20 keV - 10 MeV range. In the terms of Epeak, the burst is within 1% hardest GRBs in the sample. All the quoted errors are at the 90% confidence level. All the quoted values are preliminary.